Solid state polymerization of porous pills made by compacting polyester prepolymers

ABSTRACT

High molecular weight polyester resins such as polyethylene terephthalate, are sometimes produced from low molecular weight polyester prepolymers having the same composition by solid state polymerization. The low molecular weight polyester prepolymers which are used in such solid state polymerizations are normally prepared by conventional melt polymerizations and are generally in the form of pellets or chips. Solid state polymerization rates can be greatly improved by utilizing polyester prepolymers which are in the form of porous pills. By utilizing polyester prepolymers that are in the form of porous pills, higher molecular weights can be achieved after shorter solid state polymerization times. 
     This invention relates to an improved process for solid state polymerizing porous pills which are made by compacting fine fibers of the polyester prepolymer. The present invention specifically relates to a process for making porous pills by compacting fibers of polyester prepolymer which have a diameter of less than about 841 microns.

This is a continuation-in-part of U.S. application Ser. No. 07/341,196,filed on Apr. 21, 1989, which was a divisional of U.S. application Ser.No. 07/229,680, filed on Aug. 3, 1988 (now issued as U.S. Pat. No.4,849,497).

BACKGROUND OF THE INVENTION

High molecular weight polyesters are commonly produced from lowmolecular weight polyesters of the same composition by solid statepolymerization. The low molecular weight polyesters which are used insuch solid state polymerizations can be prepared by conventional meltpolymerizations. Solid state polymerization is generally consideredadvantageous in that the handling of high molecular weight ultra-highviscosity molten polymers is eliminated during the solid statepolymerization phase. Thermal degradation during the solid state portionof the polymerization is also essentially avoided. By utilizing solidstate polymerization techniques significant cost savings can often berealized and the level of undesirable reaction by-products, such asacetaldehydes, can be greatly reduced.

In melt polymerizations, molecular weight increases by esterification aswell as transesterification. During the initial stages of meltpolymerizations which utilize diacids, such as terephthalic acid,esterification reactions are predominate. Transesterification reactionsare predominant during the initial stages of melt polymerizations whichutilize diesters of diacids, such as dimethyl terephthalate. As themolecular weight of the polyester increases by esterification, most ofthe carboxyl end groups are consumed by esterification and water iseliminated. In transesterification reactions, alkyl end groups areconsumed with alcohols being produced as reaction by-products. Forexample, the transesterification of dimethyl terephthalate results inthe production of methanol as a reaction by-product. The final stages ofsuch polymerizations are predominated by polycondensation reactions withthe generation of glycol as a reaction by-product. The attainment ofhigh molecular weights in melt polymerizations is hindered by a reducedrate of diffusion of the glycol due to the increased viscosity of themolten polymer.

In the solid state polymerization of standard pellets or chips having arelatively low surface area per unit weight, the polymerization reactionproceeds primarily by esterification with the diffusion of water.Transesterification leads to slower solid state polymerization ratesbecause glycol by-products diffuse from the pellets or chips more slowlythan water. However, polyester prepolymer in the form of powders can besolid state polymerized at a fast rate because of the high surface areaand reduced path for glycol removal.

The low molecular weight polyester prepolymers utilized in solid statepolymerizations are generally in the form of pellets, chips, or finelydivided powder. Such pellets can vary greatly in size; however, as ageneral rule, the smaller the size of the pellets of polyesterprepolymer the faster the solid state polymerization will proceed. Suchpolyester prepolymers are generally converted from the amorphous to thecrystalline state prior to solid state polymerization in order to raisetheir sticking temperature. This is done in order to keep the pellets orchips of polyester prepolymer from sticking together as a solid mass inthe solid state polymerization reactor.

In the solid state polymerization of a polyester prepolymer thepolymerization is carried out at an elevated temperature which is belowthe melting point of the polyester resin. Such polymerizations arenormally conducted in the presence of a stream of inert gas or under avacuum. Solid state polymerizations are normally conducted on acommercial basis in the presence of a stream of inert gas since itserves to remove volatile reaction products, such as water, glycols, andacetaldehydes, and helps to heat the polyester.

Heretofore, the form of the polyester prepolymer has essentiallydictated the type of solid state polymerization process which could beemployed in order to convert the low molecular weight polyesterprepolymer into high molecular weight polyester resin. For example, ithas generally been accepted practice to solid state polymerize polyesterprepolymer in the form of pellets or chips in vacuum or in an inert gasstream using a batch or a continuous process. Fluidized bed processesare generally the best means to solid state powders of polyesterprepolymer. The reason for this is that experience has shown that finelyground powders tend to agglomerate in vacuum processes, resulting inslower polymerization rates and a need to regrind the high molecularweight polyester resin produced. Experience has also shown that, instatic bed and moving bed processes finely ground powders will channelor fissure, resulting in uneven polymerization and prolongedpolymerization rates. On the other hand, the use of pellets or chips influidized bed processes is not economically feasible in view of thevelocity and volume of inert gas needed to suspend the pellets or chipsand the size of the equipment required to do so.

Polyester prepolymers which are in the form of finely divided powderssolid state polymerize at faster rates than do polyester prepolymerswhich are in the form of pellets or chips. However, polyesterprepolymers which are in powder form are difficult to handle andgenerally must be polymerized in fluidized bed processes. Additionally,the high molecular weight polyester resins which are made utilizingprepolymers which are in powder form are also in the form of powderswhich are more difficult to process into articles of manufacture. Forthese reasons polyester prepolymers in powder form have not been widelyutilized in commercial solid state polymerization techniques.

U.S. Pat. Nos. 4,755,587 and 4,876,326 indicate that polyesterprepolymers in the form of porous pills can be solid state polymerizedat very fast polymerization rates with the high molecular weight polymerproduced having a very narrow molecular weight distribution. In fact,polyester prepolymers in the form of porous pills can be solid statepolymerized at a rate which is essentially equivalent to the rate atwhich powdered prepolymers can be solid state polymerized. Suchpolyester prepolymers in the form of porous pills can be solid statepolymerized in virtually any type of reaction zone, such as a static bedor a fluidized bed. The resultant high molecular weight polyester resinproduced can be processed in conventional equipment which is designed toaccept standard pellets or chips of the high molecular weight polyesterresin. By solid state polymerizing porous pills of polyester prepolymeressentially all of the advantages associated with using pellets or chipsare realized without being subjected to slow polymerization rates.

SUMMARY OF THE INVENTION

The subject invention discloses an improved process for making porouspills which can be solid state polymerized into high molecular weightpolyester resins. By utilizing the process of this invention porouspills can be made easily and at a low cost on a commercial basis.Additionally, porous pills made by the process of this invention can besolid state polymerized and processed into articles of manufacture witha relatively small amount of fines being generated.

The present invention more specifically reveals in a process for solidstate polymerizing low molecular weight polyester prepolymer to highmolecular weight polyester resin at an elevated temperature, theimprovement which comprises utilizing polyester prepolymer which is inthe form of porous pills which are made by compacting fibers of thepolyester prepolymer which have a diameter of less than about 841microns.

This invention further reveals a porous pill which can be solid statepolymerized at a very rapid rate into a high molecular weight polyesterresin wherein said porous pill is comprised of a polyester prepolymerhaving an intrinsic viscosity of at least about 0.2 dl/g as measured ina 60:40 phenol:tetrachloroethane solvent system at a temperature of 30°C. and at a concentration of 0.4 g/dl: wherein said porous pill containsa substantial volume of interstices which penetrate the porous pill andopen to the outside; and wherein the porous pill is made by compactingfibers of the polyester prepolymer which have a diameter of less thanabout 841 microns.

DETAILED DESCRIPTION OF THE INVENTION

This invention is applicable to virtually any polyester which can besolid state polymerized. The most common type of polyesters which willbe solid state polymerized using the technique of this invention willhave at least about 75 mole percent of their acid moieties beingprovided by an aromatic dicarboxylic acid, such as terephthalic acid,isophthalic acid, or a naphthalinic dicarboxylic acid (preferably 2,6-)with their diol moieties being provided by glycols such as ethyleneglycol, butylene glycol, 1,4-dimethylol cyclohexane and the like oraromatic diols such as hydroquinone and catechol. Such polyesters canalso contain other dicarboxylic acids such as adipic acid, isophthalicacid, sebacic acid, and the like. Polyethylene terephthalate (PET),polyethylene isophthalate, polyethylene naphthalate, and polybutyleneterephthalate homopolymers are some representative examples of suchpolyesters that can be solid state polymerized to high molecularweights. Blends of various polyesters can also be polymerized using theprocess of this invention. For instance, it can be utilized in solidstate polymerizing melt blends of polyethylene terephthalate andpolyethylene isophthalate. Such blends of polyethylene terephthalate andpolyethylene isophthalate have excellent gas barrier properties and arevery useful in making packaging materials, such as beverage bottles (seeU.S. Pat. No. 4,551,368).

The polyester prepolymers (starting polyesters) utilized in thisinvention can be made in any manner but are typically prepared byconventional melt polymerization techniques. Thus, conventionaltemperatures, catalysts, amounts of catalysts, stabilizers, and thelike, are utilized in manners well known in the literature and art inmaking the polyester prepolymer. These polyester prepolymers have aninitial starting IV (intrinsic viscosity) of at least about 0.2 dl/g asmeasured in a 60:40 phenol:tetrachloroethane solvent system at atemperature of 30° C. and at a concentration of 0.4 g/dl. The polyesterprepolymers which are solid state polymerized in accordance with thisinvention will generally have an original or starting IV of from about0.25 to about 0.65 dl/g. Preferably, the polyester prepolymer used willhave a starting IV of 0.3 to 0.4 dl/g. Virtually any type of polyesterprepolymer can be solid state polymerized using the technique of thisinvention. Such polyester prepolymers are comprised of one or morediacid components and one or more diol components.

The diacid component in the polyesters to which this invention pertainsare normally alkyl dicarboxylic acids which contain from 4 to 36 carbonatoms, diesters of alkyl dicarboxylic acids which contain from 6 to 38carbon atoms, aryl dicarboxylic acids which contain from 8 to 20 carbonatoms, diesters of aryl dicarboxylic acids which contain from 10 to 22carbon atoms, alkyl substituted aryl dicarboxylic acids which containfrom 9 to 22 carbon atoms, or diesters of alkyl substituted aryldicarboxylic acids which contain from 11 to 22 carbon atoms. Thepreferred alkyl dicarboxylic acids will contain from 4 to 12 carbonatoms. Some representative examples of such alkyl dicarboxylic acidsinclude glutaric acid, adipic acid, pimelic acid, and the like. Thepreferred diesters of alkyl dicarboxylic acids will contain from 6 to 12carbon atoms. A representative example of such a diester of an alkyldicarboxylic acid is azelaic acid. The preferred aryl dicarboxylic acidscontain from 8 to 16 carbon atoms. Some representative examples of aryldicarboxylic acids are terephthalic acid, isophthalic acid, andorthophthalic acid. The preferred diesters of aryl dicarboxylic acidscontain from 10 to 18 carbon atoms. Some representative examples ofdiesters of aryl dicarboxylic acids include diethyl terephthalate,diethyl isophthalate, diethyl orthophthalate, dimethyl naphthalate,diethyl naphthalate and the like. The preferred alkyl substituted aryldicarboxylic acids contain from 9 to 16 carbon atoms and the preferreddiesters of alkyl substituted aryl dicarboxylic acids contain from 11 to15 carbon atoms.

The diol component utilized in preparing the polyester prepolymers usedin the process of this invention is normally comprised of glycolscontaining from 2 to 12 carbon atoms, glycol ethers containing from 4 to12 carbon atoms, and polyether glycols having the structural formulaHO--A--O)_(n) H wherein A is an alkylene group containing from 2 to 6carbon atoms and wherein n is an integer from 2 to 400. Generally, suchpolyether glycols will have a molecular weight of 400 to about 4000.

Preferred glycols normally contain from 2 to 8 carbon atoms withpreferred glycol ethers containing from 4 to 8 carbon atoms. Somerepresentative examples of glycols that can be utilized as the diolcomponent include ethylene glycol, 1,3-propylene glycol, 1,2-propyleneglycol, 2,2-diethyl-1,3-propane diol, 2,2-dimethyl-1,3-propane diol,2-ethyl-2-butyl-1,3-propane diol, 2-ethyl-2-isopbutyl-1,3-propane diol,1,3-butane diol, 1,4-butane diol, 1,5-pentne diol. 1,6-hexane diol,2,2,4-trimethyl-1,6-heane diol, 1,3-cyclohexane dimethanol,1,4-cyclohexane dimethanol, 2,2,4,4-tetramethyl-1,3-cyclobutane diol,and the like. Some representative examples of polyether glycols that canbe used include polytetramethylene glycol (Polymeg™) and polyethyleneglycol (Carbowax™).

Branched polyester prepolymers can also be solid state polymerized inaccordance with the process of the present invention. Such branchedpolyesters normally contain branching agents which contain three or morefunctional groups and preferably contain three or four functionalgroups. Such reactive functional groups can be carboxyl groups oraliphatic hydroxyl groups. The branching agent utilized in such branchedpolyesters can optionally contain both carboxyl groups and hydroxylgroups. Examples of acidic branching agents include trimesic acid,trimellitic acid, pyromellitic acid, butane tetracarboxylic acid,naphthalene tricarboxylic acids, cyclohexane-1,3,5-tricarboxylic acids,and the like. Some representative examples of hydroxyl branching agents(polyols) include glycerin, trimethylol propane, pentaerythritol,dipentaerythritol, 1,2,6-hexane triol, and 1,3,5-trimethylol benzene.Generally, from 0 to 3 percent of a polyol containing from 3 to 12carbon atoms will be used as the branching agent (based upon the totaldiol component).

High strength polyesters which utilize at least one bis-hydroxyalkylpyromellitic diimide in their diol component can also be solid statepolymerized in the form of porous pills. The diol component in such highstrength polyesters will normally contain from 5 to 50 mole percent ofone or more bis-hydroxyalkyl pyromellitic diimides and will preferablybe comprised of from 10 to 25 mole percent of at least onebis-hydroxyalkyl pyromellitic diimide. The remaining portion of the diolcomponent in such high strength polyesters is comprised of additionalcopolymerizable diols. Such high strength polyesters are described ingreater detail in U.S. Pat. No. 4,605,728, which is incorporated hereinby reference in its entirety.

In accordance with the present invention, the low molecular weightpolyester prepolymer is melt spun into fine fibers which have a diameterof less than about 841 microns. It is desirable for the fibers ofpolyester prepolymer to have a diameter which is within the range ofabout 25 to about 500 microns. In most cases it will be preferred forthe polyester prepolymer fibers to have a diameter which is within therange of about 50 microns to about 250 microns. These fibers ofpolyester prepolymer can be melt spun using standard melt spinningtechniques which are well known to persons skilled in the art. Forexample, molten polyester prepolymer can be extruded or pumped throughspinnerets of the proper size to produce fibers having the desireddiameter. In most cases, it will be desirable to strain or filter themolten prepolymer to protect the spinning system from melt impuritiesand to improve the quality of the high molecular weight resin beingmade. The fibers spun are normally air cooled to a temperature which isbelow the melting point of the polyester prepolymer (quenched). This canbe done utilizing blast nozzles which have adjustable air streamguidance. It is sometimes desirable to utilize a suction device tocollect spinning fumes to ensure odorless working conditions. In manycases it will be highly desirable to cut the fibers before compactingthem into porous pills. For example, the fibers can be cut utilizing atow cutter which is equipped with a cutting reel which is typicallyprovided with a number of throw-away blades around its circumference(the fiber length being determined by the spacing of the blades). Porouspills can then be made utilizing such fibers of the polyesterprepolymer.

The porous pills can be made by simply compacting the fibers of thepolyester prepolymer into porous pills. For example, porous pills whichare suitable for use in the process of this invention can be made bysimply compacting the fibers of polyester prepolymer having a diameterof less than about 841 microns utilizing a high pressure of at leastabout one ton per square inch (1.379×10⁷ Pascals). Pressures within therange of about 1 to about 40 tons per square inch (1.379×10⁷ to5.516×10⁸ Pascals) are appropriate for compacting the fibers into porouspills. It is, of course, also possible to utilize even higher pressuresin forming the porous pills. Porous pills can be made in this mannerutilizing tablet presses which are similar to those used in thepharmaceutical industry. The temperature at which the porous pills areprepared by compaction is not critical but will be below the meltingpoint of the polyester prepolymer. However, in most cases the porouspills will be prepared by compaction at a temperature within the rangeof about -20° C. to about 150° C. with temperatures of 10° C. to 50° C.being more common. It is appropriate to prepare the porous pills bycompaction at room temperature, simply for convenience.

Porous pills can be made utilizing compaction/granulation systems. Insuch a technique, the fibers of polyester prepolymer are compactedbetween compaction rolls into a sheet which is subsequently granulatedinto porous pills which are in the form of granules. In a similartechnique, porous pills can be made utilizing briquetting systems. Insuch briquetting systems, the fibers of polyester prepolymer arecompacted between compaction rolls into porous pills in the form ofbriquettes.

Porous pills can also be made by agglomerating fibers of the polyesterprepolymer. For example, fibers of the prepolymer can be stuck togetherat a temperature which is above the sticking point of the polyesterprepolymer into the form of porous pills. It is important to avoidexcessive heat build-up which can destroy the porous nature of theporous pills being made.

A binder material can be used in agglomerating the fibers into porouspills. The binder material used will be capable of sticking the fiberstogether at a very low concentration. However, the binder will becompatible with the end use for which the high molecular weightpolyester resin being made is intended. For example, the use of binderssuch as starch can result in color formation which is not desirable inhigh molecular weight polyester resin which is intended for use inmaking clear beverage bottles.

Satisfactory porous pills of virtually any size or shape can beprepared. However, the porous pills will normally weigh from about0.0005 grams to about 0.5 grams. The preferred weight for the porouspills will depend upon the method by which they are manufactured. Thisis because some manufacturing techniques are very appropriate for makinglarge porous pills with others being more suitable for making smallporous pills. For example, porous pills which are made on a tablet presspreferably weigh from about 0.03 grams to about 0.2 grams and morepreferably weigh from 0.05 grams to 0.1 grams. Porous pills which aremade utilizing a compaction/granulation process will preferably weighfrom about 0.001 grams to about 0.1 grams and will more preferably weighfrom about 0.005 grams to about 0.015 grams. The porous pills made canhave virtually any shape. For instance, the porous pills can be in theform of cubes, cylinders, discs, spheres, rods, briquettes, or granules.It is desirable to prepare porous pills which are cylindrical or discshaped in cases where the porous pills are made on a tablet press. Thisis because cylinders and discs can be easily prepared utilizing tabletpresses and have good structural integrity. Irregular shaped granulescan be easily made and are preferred in cases where the porous pills aremade by a compaction/granulation process.

The density of the porous pills utilized in accordance with the processof this invention is naturally lower than the density of pellets orchips which are comprised of the same polyester having the samecrystallinity. For instance, solid pellets which are comprised ofpolyethylene terephthalate prepolymer have a density that ranges fromabout 1.33 grams per cubic centimeter at a crystallinity of 0% to adensity of about 1.46 grams per cubic centimeter at a crystallinity of100%. Porous pills which are comprised of polyethylene terephthalateprepolymer typically have a density within the range of 1.0 to 1.3 gramsper cubic centimeter at a crystallinity of 25%. Standard polyethyleneterephthalate chips have a density of about 1.36 grams per cubiccentimeter at a crystallinity of 25%. The porous pills of this inventionhave a density which is within the range of about 70% to about 99% ofthe density of solid polyester pellets which are comprised of the samepolyester prepolymer having the same crystallinity. In most cases porouspills which are comprised of a given polyester prepolymer will have adensity within the range of 70% to 96% of the density of solid polyesterpellets which are comprised of the same polyester prepolymer. It isgenerally preferred for such porous pills to have a density of 85% to93% of the density of solid polyester pellets which are comprised of thesame polyester prepolymer. The porous pills of this invention have lowerdensities because they contain a significant amount of void space. Thevoid space in the porous pills of this invention is comprised of asubstantial amount of open pores or cavities which penetrate the porouspills. These cavities form passage ways through which water, glycols,acetaldehydes, and other reaction by-products which are formed in thesolid state polymerization can more readily escape from the inner mostportions of the porous pills. In other words, the porous pills contain asubstantial volume of interstices which penetrate the porous pills andopen to the outside.

The polyester prepolymer in the porous pills can optionally be convertedto a higher degree of crystallinity prior to solid state polymerizationin order to raise the sticking temperature of the porous pills. This isdone in order to reduce the possibility of the porous pills stickingtogether as a solid mass in the solid state polymerization zone.Preferably, the surfaces of the polyester pills being solid statepolymerized will have a crystallinity of at least about 20%. Porouspills have less tendency to stick together if their surface iscrystallized even though they may be composed of amorphous polyester onthe inside. Increased crystallinity can be achieved by any suitabletreatment, such as by controlled heating of the porous pills or bytreating the porous pills with the vapor of a suitable organic solvent.

The sticking temperature of crystalline or partially crystallinepolyethylene terephthalate prepolymers is about 240° C. which is muchhigher than their sticking temperature in the amorphous state which isabout 100° C. to about 150° C. Polyester prepolymers are commonlycrystallized to a crystallinity of at least about 20% prior to solidstate polymerization. Typically, a polyester prepolymer can be convertedto about 20% to 40% crystallinity by heating at 150° C. for 5 minutes to300 minutes. At lower crystallization temperatures, longer heating timesare normally required. Suitable time-temperature relationships can beeasily determined by persons having skill in the art.

The amount of time required to crystallize the surfaces of porous pillsby vapor treatment will vary with the concentration of the vapor, withthe type of volatile organic compound being utilized, with the type ofpolyester prepolymer being utilized and with the crystallinity desired.Since vapor treatment crystallizes the porous pills from their surfaceinward, generally a lower degree of crystallinity is required to preventporous pills made out of the polyester prepolymer from sticking than isrequired when the porous pills are crystallized utilizing a thermaltreatment. This increased degree of crystallization in the porous pillsis attained by simply exposing the porous pills to the vapors of asuitable organic compound, such as a volatile chlorinated hydrocarbon, avolatile ketone, tetrahydrofuran, ethylene oxide, or propylene oxide.Methylene chloride and acetone are particularly preferred volatileorganic compounds for utilization in such vapor crystallizations.

The porous pills can also be crystallized in a high frequency energyfield ranging from about 20 megahertz to about 300 megahertz. Such atechnique is described in greater detail in U.S. Pat. No. 4,254,253,which is incorporated herein by reference in its entirety.

Porous pills which are comprised of polyester prepolymers can be solidstate polymerized in accordance with the process of this invention in abatch or continuous process. Suitable solid state polymerizationtemperatures can range from a temperature just above the thresholdtemperature of the polymerization reaction up to a temperature within afew degrees of the sticking temperature of the porous pills which can bewell below their melting point. For instance, the sticking temperatureof crystalline or partially crystalline polyethylene terephthalateprepolymer can be as high as about 240° C. which is less than itsmelting point of about 258° C. and amorphous polyethylene terephthalateprepolymer has a sticking temperature of only about 100° C. to about150° C. which is much less than its melting point.

The solid state polymerizations of this invention are generallyconducted at a temperature which is within the range of about 50° C.below the sticking temperature of the polyester prepolymer up to but notincluding the sticking temperature of the polyester prepolymer. Thesolid state polymerization temperature utilized will typically be fromabout 1° C. to about 50° C. below the sticking temperature of the porouspills which are comprised of polyester prepolymer. The optimum solidstate reaction temperature will differ somewhat for prepolymers ofdifferent compositions and of different molecular weights. As a generalrule, the optimum solid state polymerization temperature for porouspills will be from about 5° C. to about 20° C. below their stickingtemperature. For example, in the solid state polymerization of porouspills comprised of polyethylene terephthalate, the highest temperatureswhich can normally be employed range from 240° C. to about 255° C. whichis just below the sticking temperature and melting point of thispolyester. Generally, the polyethylene terephthalate will be solid statepolymerized at a temperature of from about 210° C. to about 255° C. Inmost cases, polyethylene terephthalate will be solid state polymerizedat a temperature of from 230° C. to 250° C.

As the solid state polymerization of a polyester prepolymer proceeds,its sticking temperature can increase. Thus, the solid statepolymerization temperature can be incrementally increased during thecourse of the polymerization. For example, in the case of polyethyleneterephthalate the process described in U.S. Pat. No. 3,718,621, which isincorporated herein by reference in its entirety, can be utilized.

The solid state polymerization is conducted under a vacuum or in thepresence of a stream of an inert gas. Normally such solid statepolymerizations are conducted in the presence of an inert gas stream. Itis highly desirable for the inert gas to flow uniformly throughout thesolid state polymerization zone which is filled with porous pills whichare comprised of polyester prepolymer which is being polymerized. Inorder to help insure that the inert gas flows homogeneously or uniformlythrough the solid state polymerization zone without bypassing certainareas in it, a device for dispersing the inert gas is generally used.Thus, a good polymerization reactor will be designed in such a way thatthe inert gas will flow homogeneously through the porous pills in it. Itshould be noted that most of the inert gas flows around the porous pillsof polyester prepolymer as it streams through the solid statepolymerization zone.

Some suitable inert gases for use in the solid state polymerizationprocess of this invention include nitrogen, carbon dioxide, helium,argon, neon, krypton, xenon, and certain industrial waste gases. Variouscombinations or mixtures of different inert gases can also be used. Inmost cases nitrogen will be used as the inert gas.

The total amount of inert gas needed to solid state polymerize apolyester prepolymer into a high molecular weight polyester resin in agiven solid state polymerization reactor at a given temperature can begreatly reduced by pulsing the inert gas through the polyesterprepolymer. The minimum amount of inert gas that needs to be pulsedthrough a given polyester prepolymer per unit time per unit weight willvary with the polyester, the solid state polymerization temperatureused, and the design of the polymerization reactor. The optimum mannerof pulsing the inert gas through the porous pills will also vary withthe type of polyester used, the polymerization temperature used, and thedesign and size of the polymerization reactor. Usually, between about0.05 and about 2 liters of inert gas per hour will be pulsed through thepolyester prepolymer per kilogram of the polyester prepolymer. Generallythe best way to pulse the inert gas through the polyester prepolymerbeing solid state polymerized is to repeatedly turn the flow of inertgas on and off. U.S. Pat. No. 4,532,319, which is incorporated herein byreference in its entirely describes this technique for reducing theconsumption of inert gas in greater detail.

The solid state polymerization zone can be of virtually any design thatwill allow the polyester prepolymer to be maintained at the desiredsolid state polymerization temperature for the desired residence timeand which allows for removal of reaction by-products, such asacetaldehyde. Such solid state polymerization zones can be reactorswhich have a fixed bed, a static bed, a fluidized bed, or a moving bed.In most cases, it is preferred to utilize a tubular polymerizationreactor wherein the porous pills flow through the reactor for thedesired residence time. Such tubular reactors have a substantiallyuniform cross-section and a sufficient height to allow the porous pillsto flow by reason of the force of gravity from the top to the bottom ofthe reactor in the desired residence time. In other words, the porouspills move from the top to the bottom of such a tubular polymerizationreactor in a partially dammed state. The rate of flow through such areactor can be controlled by regulating discharge at the bottom of thereactor. It is generally preferred to allow an inert gas to flowcountercurrently (upwardly) through the reactor at a gas velocity wellbelow the turbulence point so that the porous pills are not fluidized(always remain in contact with each other). The porous pills remain insubstantially the same physical form throughout the solid statepolymerization process.

The polyester prepolymer will be solid state polymerized for a timesufficient to increase its molecular weight or IV to that of the highmolecular weight polyester resin desired. It will be desirable for thehigh molecular weight polyester resin being prepared to have an IV of atleast about 0.65 dl/g. In most cases the high molecular weight resinwill have an IV of at least 0.7 dl/g and for many applications willpreferably have an IV of at least about 1.0 dl/g. Polyester resinshaving an IV of greater than 2.0 dl/g are useful in some application andcan be made by using the process of this invention.

The polymerization time required to increase the molecular weight of thepolyester prepolymer to a given desired molecular weight will, ofcourse, vary with the polymerization temperature used, the IV of thepolyester prepolymer utilized, and with the final IV desired. Very longpolymerization times can be utilized if ultra-high molecular weightpolyester resins are desired. In most cases the polymerization timesutilized in carrying out the process of this invention will range fromabout 30 minutes to about 24 hours. In making polyethylene terephthalateresin having an IV within the range of 0.65 dl/g to 0.85 dl/g from aprepolymer having an IV of less than 0.3 dl/g, it will be preferred toutilize a polymerization time ranging between about 1 hour and about 12hours. Such solid state polymerizations can, of course, be carried oututilizing batch, semi-continuous, or continuous techniques. In the caseof continuous solid state polymerizations, the polymerization timesreferred to are residence times in the solid state polymerizationreaction zone.

The rate at which porous pills made from polyethylene terephthalateprepolymer can be solid state polymerized will also depend upon thecarboxyl end group content of the prepolymer. As a general rule,polyethylene terephthalate prepolymers having a minimal carboxyl contentachieve a maximum solid state polymerization rate in the polymerizationsof this invention. Thus, for maximum solid state polymerization rates apolyethylene terephthalate prepolymer having a carboxyl end groupcontent of less than about 30% and preferable less than about 20% can beused. However, in some cases it will not be desirable to usepolyethylene terephthalate prepolymers with such low carboxyl contentsbecause high molecular weight resins having low carboxyl contents arenot desirable in some applications. In such cases polyethyleneterephthalate prepolymer having higher carboxyl end group contents can,of course, be used.

This invention is illustrated by the following examples which are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise all parts and percentages aregiven by weight.

EXAMPLE 1

Polyethylene terephthalate prepolymer having an IV of 0.25 dl/g wasprepared utilizing a conventional melt polymerization technique. Thepolyethylene terephthalate prepolymer was melt spun into fibers having adiameter of less than 250 microns. The fibers were then cut by hand intostrands which varied in length from about 1/32 inch (0.8 mm) to about1/4 inch (6.4 mm). The fibers were then compacted into porous pillsusing a pellet press having a 0.5 inch (12.7 mm) diameter die.

The porous pills were dried and crystallized at a temperature of about356° F. (180° C.) for 1 hour. A laboratory size solid statepolymerization reactor, equipped with a sintered glass dispersing plate,was then utilized in solid state polymerizing the porous pills whichwere made. The porous pills were put into the reactor and the reactorwas placed in a constant temperature bath which was maintained at about456° F. (236° C.). A preheated nitrogen stream was allowed to flowthrough the solid state polymerization reactor at a constant flow rateof about 12 standard cubic feet (0.35 m³) per hour. The polyethyleneterephthalate prepolymer was allowed to solid state polymerize for about6 hours and 20 minutes with the IV of the polyester being measured after2 hours, 4 hours, 51/3 hours and 61/3 hours. After 2 hours the polyesterhad an IV of about 0.62 dl/g. It had an IV of about 0.77 dl/g after 4hours and an IV of about 0.86 dl/g after 51/3 hours. The high molecularweight polyethylene terephthalate resin produced by this process wasdetermined to have an IV of about 0.893 dl/g after 61/3 hours. Thisexample clearly shows that the process of this invention can be used toprepare polyester resins having very high molecular weights frompolyester prepolymers having very low molecular weights while utilizingrelatively short polymerization times. The process of this invention canbe utilized to greatly increase the through put of a given solid statepolymerization reactor. Accordingly, the process of this inventiongreatly reduces the energy requirements required in order to increasethe IV of a polyester prepolymer to a given higher IV which is desired.

This example also shows that the porous pills of this invention havingan IV of less than 0.3 dl/g can be solid state polymerized to an IV ofgreater than 0.8 dl/g in less than 6 hours. In this example the solidstate polymerization resulted in the polyester increasing in IV at anaverage rate of greater than 0.1 dl/g per hour.

Variations in the present invention are possible in light of thedescription of it provided herein. It is, therefore, to be understoodthat changes can be made in the particular embodiments described whichwill be within the full intended scope of the invention as defined bythe following appended claims.

What is claimed is:
 1. A porous pill which can be solid statepolymerized at a very rapid rate into a high molecular weight polyesterresin wherein said porous pill is comprised of a polyester prepolymerhaving an intrinsic viscosity of at least about 0.2 dl/g as measured ina 60:40 phenol:tetrachloroethane solvent system at a temperature of 30°C. and at a concentration of 0.4 g/dl: wherein said porous pill containsa substantial volume of interstices which penetrate the porous pill andopen to the outside; and wherein the porous pill is made by compactingfibers of the polyester prepolymer which have a diameter of about 25microns to about 841 microns and a length of at least 0.08 mm.
 2. Aporous pill as specified in claim 1 wherein the polyester prepolymer ispolyethylene terephthalate.
 3. A porous pill as specified in claim 1wherein the polyester prepolymer is polyethylene naphthalate.
 4. Aporous pill as specified in claim 1 wherein the fibers have a diameterof about 25 microns to about 500 microns.
 5. A porous pill as specifiedin claim 2 wherein the fibers have a diameter of about 50 microns toabout 250 microns.
 6. A porous pill as specified in claim 2 wherein thepolyethylene terephthalate has an intrinsic viscosity which is withinthe range of about 0.25 dl/g to about 0.65 dl/g.
 7. A porous pill asspecified in claim 2 wherein the polyethylene terephthalate has anintrinsic viscosity which is within the range of about 0.3 to about 0.4dl/g.
 8. A porous pill as specified in claim 2 wherein the porous pillis made utilizing a compaction/granulation system.
 9. A porous pillwhich can be solid state polymerized at a very rapid rate into a highmolecular weight polyester resin wherein said porous pill is comprisedof a polyester prepolymer having an intrinsic viscosity of at leastabout 0.2 dl/g as measured in a 60:40 phenol:tetrachloroethane solventsystem at a temperature of 30° C. and at a concentration of 0.4 g/dl;wherein said porous pill contains a substantial volume of intersticeswhich penetrate the porous pill and open to the outside: and wherein theporous pill is made by agglomerating fibers of the polyester prepolymerwhich have a diameter of about 25 microns to about 841 microns and alength of at least 0.8 mm.
 10. A porous pill as specified in claim 9wherein the fibers are agglomerated by sticking them together at atemperature above their sticking temperature and forming them into theshape of the porous pills.
 11. A porous pill as specified in claim 8wherein the porous pill weighs from about 0.005 g to about 0.015 g. 12.A porous pill as specified in claim 8 wherein the porous pill weighsfrom about 0.001 g to about 0.1 g.
 13. A porous pill as specified inclaim 2 wherein the fibers are compacted at a temperature which iswithin the range of about 10° C. to about 50° C.
 14. A porous pill asspecified in claim 2 wherein the fibers are compacted utilizing apressure of from about 1 ton per square inch to about 40 tons per squareinch.
 15. A porous pill as specified in claim 2 wherein the porous pillhas a density which is within the range of about 70% to about 96% of thedensity of solid polyester pellets which are comprised of the samepolyester prepolymer having the same crystallinity.
 16. A porous pill asspecified in claim 2 wherein the porous pill has a density which iswithin the range of about 85% to about 93% of the density of solidpolyester pellets which are comprised of the same polyester prepolymerhaving the same crystallinity.
 17. A porous pill as specified in claim 2wherein the porous pill has a density which is within the range of about70% to about 99% of the density of solid polyester pellets which arecomprised of the same polyester prepolymer having the samecrystallinity.
 18. A porous pill as specified in claim 9 wherein thepolyester prepolymer is polyethylene terephthalate.
 19. A porous pill asspecified in claim 18 wherein the fibers have a diameter of about 25microns to about 500 microns; and wherein the polyethylene terephthalatehas an intrinsic viscosity which is within the range of about 0.25 dl/gto about 0.65 dl/g.
 20. A porous pill as specified in claim 18 whereinthe fibers have a diameter of about 50 microns to about 250 microns: andwherein the polyethylene terephthalate has an intrinsic viscosity whichis within the range of about 0.3 dl/g to about 0.4 dl/g.